<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        div {
            border: 2px red solid;
        }

        #box3 {
            width: 200px;
            height: 200px;
            position: relative;
        }

        #box4 {
            width: 99%;
            position: absolute;
            bottom: 0;
            left: 0;

        }
    </style>
</head>
<body>
<div id="box3">
    <img src="http://img1.sycdn.imooc.com/541a7d8a00018cf102000200.jpg">
    <div id="box4">当我还是三年级的学生时是一个害羞的小女生。</div>
</div>
</body>
</html>
